The function of the "Gene Targeting Core" (Core C) is to facilitate gene-targeting and BAC- transgenic projects for all progrann investigators. The "Gene Targeting Core" will continue to perform all aspects of mouse embryonic stem cell culture and gene targeting, including the preparation of feeder layers, the culture and electroporation of ES cells (both 129 and C57/BL6 derived), the isolation of DNA, the expansion of targeted ES cell clones, and the preparation of targeted ES cells for blastocyst injection. We will also maintain Cre-lines for program investigators, and assist with the construction of new alleles or transgenes via BAC recombineering. In addition, we will carry out a systematic evaluation of available Cre lines for gene ablation in adipose tissue for the P01 investigators.